Reduction of noise to enhance the perceived signal-to-noise ratio is the primary function of typical audio processors applied to FM receivers. These circuits are usually applied to the detected audio signal after stereo decoding has taken place. The three most commonly applied audio processing mechanisms are blend, rolloff, and audio attenuation (or mute). Blend is the reduction in L-R content, rolloff is the reduction of high-frequency content (where noise is not easily masked by program material), and audio attenuation is the reduction of the overall audio signal level in a gradual squelch-type action.
As signal conditions degrade, these mechanisms are applied in this order to minimize the disruptive effect as perceived by the listener. Typically, blend is applied when the RF signal drops below a level of 50 to 100 uV or during slight signal disturbances, while rolloff and attenuation are best applied only as conditions degrade severely.
The application of audio processing is very dependent upon the response of the FM receiver to multipath interference and loss of signal. In particular, the noise output of the detector under weak signal conditions dictates the threshold of both rolloff and attenuation. This is due to the relative steepness of the S/N curve vs. RF input for the FM detector. As the signal level is reduced, the noise characteristic can be divided into three distinct regions as influenced by receiver noise floor, thermal noise at the antenna, and detector-generated (capture) noise. The graph shown in FIG. 1, illustrates this behavior.
Blend is typically applied as soon as the thermal noise at the antenna begins to limit the stereo S/N. It should be noted that the stereo S/N ratio is about 23 dB worse than mono at this point. Rolloff is best applied at a point below, say, 10 uV, where the detector is contributing the majority of the noise power. Finally, when the overall S/N ratio collapses and the noise floor rises to within 20 dB or so of the full-strength detected audio, an audio attenuation action is desirable to keep the overall noise volume below a point of annoyance to the listener.
Depending on the slope of the FM detector noise characteristic, the need for an audio attenuator varies. With certain receivers, the slope is much gentler and the point of convergence of the two curves (detected audio and noise) is at a lower audio level. In such receivers the need for an audio attenuator has not been great. However, hard-limiting switched-capacitor type FM detectors have much steeper noise slopes and need an audio attenuator to enhance listenability under weak or no signal conditions. As a subjective rule, the level of the noise floor (which increases as signal strength decreases) should remain at least 22 dB below full quieting 100% modulated audio. In accordance with the present invention a closed-loop audio attenuator circuit is provided that insures that these concerns are met.